1. Technical Field
The present invention relates generally to liquid crystal devices and, in particular, to a liquid crystal device driver circuit for electrostatic discharge protection.
2. Description of Related Art
In general, a liquid crystal device (hereinafter referred to as xe2x80x9cLCDxe2x80x9d) driver circuit or an integrated circuit (hereinafter referred to as xe2x80x9cICxe2x80x9d) drives a high-level LCD voltage (VLCD) to display information on an LCD panel. Here, the LCD voltage (VLCD) can be externally applied and internally generated using an analog circuit such as an internal charge pump, an operational amplifier, or a band gap circuit. The VLCD is an important factor of the picture quality of an LCD screen.
However, internal circuits in an LCD driver circuit can be damaged by an electrostatic discharge (hereinafter referred to as xe2x80x9cESDxe2x80x9d) phenomenon occurring in a voltage input port or a voltage output port. Thus, most semiconductor devices as well as the LCD driver circuit include devices for ESD protection on an input port or output port to protect the semiconductor devices from damage by the ESD phenomenon.
FIG. 1 is a circuit diagram of a conventional LCD driver circuit for ESD protection. The circuit shown in FIG. 1 is an example of a conventional driver circuit applied in a monochrome LCD and includes an input pad 10, a resistor R1, an ESD protection unit 12, a voltage generating unit 14, and an LCD output driver 16.
In the circuit shown in FIG. 1, LCD voltages (VLCDs) V1 through V5 are externally applied through each input pad, and high-level voltage is divided by the voltage generating unit 14 to generate the VLCDs V1 through V5. Although not specifically shown, second through fifth voltages V2 through V5 can be applied to the LCD output driver 16 by the same method as that used for a first voltage V1. During a normal operation, the ESD protection unit 12 does not operate. However, when an ESD pulse is applied through the input pad 10, the serial resistor R1 and a first protection device D1 or a second protection device D2 are turned on to form a discharge path for discharging a high current of the ESD pulse. Here, the high current of the ESD pulse is lowered by the serial resistor R1 connected to the input pad 10, to protect the internal circuits.
However, the amount of change of the LCD voltages (VLCDs) in the LCD driver circuit for driving a color LCD other than the monochrome LCD is strictly stipulated in its design specification. For example, under specific test conditions, when a difference between a current flowing into the pad 10 to which the LCD voltages (VLCDS) are input and a current flowing into the internal voltage generating unit 14 is 10 uA, the amount of change of the VLCDs is less than 10 mV. Thus, in the color LCD driver circuit, other than the circuit of FIG. 1, a serial resistor which is a main factor of voltage drop cannot be connected between an input pad and a voltage generating unit. As a result, the high current of the ESD pulse is transferred to the output driver 16 and the voltage generating unit 14, thereby causing physical damage. That is, when the ESD pulse with positive polarity or negative polarity is applied, first discharge is performed by the first and second protection devices D1 and D2 of the ESD protection unit 12 adjacent to the pad 10, and a remaining current is applied to the LCD output driver 16.
FIG. 2 a circuit diagram of an output driver applied in a conventional color LCD driver circuit. Each voltage transferring device to which VLCDs V1 through V3 having relatively high-voltage levels are transferred, is implemented by CMOS transfer gates TG21 through TG23. The transferring devices for transferring VLCDs V4 and V5 having low voltage levels are implemented by NMOS transistors MN21 and MN22. Also, an ESD protection unit 25 is provided to protect internal circuits from an ESD pulse applied through an output pad 22. The output driver of the color LCD driver circuit is designed to satisfy on-resistance according to its design specification. In other words, on-resistance of each of the transfer gates TG21 through TG23 and the NMOS transistors MN21 and MN22 is decided in proportion to the VLCDs V1 through V5. Thus, desired on-resistance for driving the VLCDs V4 and V5 having low voltage levels is obtained only by the NMOS transistors MN21 and MN22 having a small width.
However, in the case of using the NMOS transistors, there is no forward discharge path when the ESD pulse with positive polarity is applied. Also, since the discharge area is very small, the discharge capability is very weak.
Additionally, in the conventional LCD driver circuit, since the discharge efficiency of the protection devices (for example, D1 and D2 of FIG. 1) connected to the input pads is very low, ESD protection can be deteriorated. That is, since the VLCD voltages are higher than an operating voltage of any other circuit in the LCD driver, the ESD protection unit 12 of FIG. 1 is formed of a high voltage junction. However, since the operating voltage is high in the high voltage junction, a high current is not driven. Thus, in a case where the high current due to the ESD pulse is applied, the ESD protection can be deteriorated.
To solve the above and other related problems of the prior art, there is provided a liquid crystal device (LCD) driver circuit for electrostatic discharge protection. The LCD driver circuit is capable of preventing an output driver from being damaged by an ESD pulse in a color LCD driver circuit, and improves the efficiency of protecting against electrostatic discharge.
According to an aspect of the invention, there is provided a liquid crystal device (LCD) driver circuit. The LCD driver circuit includes first through N-th input pads for respectively receiving first through N-th voltages. The first through N-th voltages have different voltage levels and are externally applied to the LCD driver circuit. N is an integer greater than one. First through N-th electrostatic discharge (ESD) protection units are respectively connected to the first through N-th input pads, and form a discharge path when an electrostatic pulse is respectively applied through any of the first through N-th input pads. An output driver has first through N-th resistors. The first through N-th resistors respectively receive the first through N-th voltages input through the first through N-th input pads. The output driver generates a driving voltage for driving an LCD from each of the first through N-th voltages received through the first through N-th resistors, respectively. The first through N-th resistors reduce a current flowing into the output driver when the electrostatic pulse is applied.
According to another aspect of the invention, there is provided a liquid crystal device (LCD) driver circuit. The LCD driver circuit includes first through N-th input pads for respectively receiving first through N-th voltages. The first through N-th voltages have different voltage levels and are externally applied to the LCD driver circuit. N is an integer greater than one. First through N-th electrostatic discharge (ESD) protection units are respectively connected to the first through N-th input pads, and form a discharge path when an electrostatic pulse is respectively applied through any of the first through N-th input pads. An output driver has first through N-th voltage transferring means. The first through N-th voltage transferring means respectively transfer the first through N-th voltages input through the first through N-th input pads, respectively. The output driver generates a driving voltage for driving an LCD from each of the first through N-th voltages transmitted through the first through N-th voltage transferring means, respectively. At least one voltage transferring means of the first through N-th voltage transferring means transfers low-level voltages of the first through N-th voltages and has a parallel structure of a PMOS transistor and an NMOS transistor.
According to yet another aspect of the invention, there is provided a liquid crystal device (LCD) driver circuit. The LCD driver circuit includes first through N-th input pads for respectively receiving first through N-th voltages. The first through N-th voltages have different voltage levels and are externally applied to the LCD driver circuit. N is an integer greater than one. First through N-th electrostatic discharge (ESD) protection units are respectively connected to the first through N-th input pads, and form a discharge path when an electrostatic pulse is respectively applied through any of the first through N-th input pads. The first through N-th ESD protection units include at least one thin gate-oxide (gox) transistor.
These and other aspects, features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments, which is to be read in connection with the accompanying drawings.